linux-kernel - Re: [LLVMdev] Cc llvmdev: Re: llvm bpf debug info. Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

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Message-ID: <55C1B254.5030801@huawei.com>
Date:	Wed, 5 Aug 2015 14:51:00 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Alexei Starovoitov <ast@...mgrid.com>,
	He Kuang <hekuang@...wei.com>, pi3orama <pi3orama@....com>
CC:	<llvm-dev@...ts.llvm.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [LLVMdev] Cc llvmdev: Re: llvm bpf debug info. Re: [RFC PATCH
 v4 3/3] bpf: Introduce function for outputing data to perf event



On 2015/8/5 10:05, Wangnan (F) wrote:
> Send again since llvmdev is moved to llvm-dev@...ts.llvm.org
>
> On 2015/8/5 9:58, Wangnan (F) wrote:
>>
>>>
>>> On 2015/8/4 3:44, Alexei Starovoitov wrote:
>>>> On 7/31/15 3:18 AM, Wangnan (F) wrote:
>>>>
>>>
>>> [SNIP]
>>>
>>>> didn't have time to look at it.
>>>> from your llvm patches looks like you've got quite experienced
>>>> with it already :)
>>>>
>>>>> I'll post 2 LLVM patches by replying this mail. Please have a look 
>>>>> and
>>>>> help me
>>>>> send them to LLVM if you think my code is correct.
>>>>
>>>> patch 1:
>>>> I don't quite understand the purpose of builtin_dwarf_cfa
>>>> returning R11. It's a special register seen inside llvm codegen
>>>> only. It doesn't have kernel meaning.
>>>>
>>>
>>> Kernel side verifier allows us to do arithmetic computation using 
>>> two local variable
>>> address or local variable address and R11. Therefore, we can compute 
>>> the location
>>> of a local variable using:
>>>
>>>   mark = &my_var_a - __builtin_frame_address(0);
>>>
>>> If the stack allocation is fixed (if the location is never reused), 
>>> the above 'mark'
>>> can be uniquely identify a local variable. That's why I'm 
>>> interesting in it. However
>>> I'm not sure whether the prerequestion is hold.
>>>
>>>> patch 2:
>>>> do we really need to hack clang?
>>>> Can you just define a function that aliases to intrinsic,
>>>> like we do for ld_abs/ld_ind ?
>>>> void bpf_store_half(void *skb, u64 off, u64 val) 
>>>> asm("llvm.bpf.store.half");
>>>> then no extra patches necessary.
>>>>

And for this:

I tried this test function:

void bpf_store_half(void *skb, int off, int val) asm("llvm.bpf.store.half");
int func()
{
         bpf_store_half(0, 0, 0);
         return 0;
}

Compiled with:

$ clang -g -target bpf -O2 -S -c test.c

And get this:

         .text
         .globl  func
         .align  8
func:                                   # @func
# BB#0:                                 # %entry
         mov     r1, 0
         mov     r2, 0
         mov     r3, 0
         call    llvm.bpf.store.half
         mov     r0, 0
         ret

Without -S, it generate a function relocation:

$ objdump -r ./test.o

./test.o:     file format elf64-little

RELOCATION RECORDS FOR [.text]:
OFFSET           TYPE              VALUE
0000000000000018 UNKNOWN           llvm.bpf.store.half


It doesn't work as you suggestion. I think we still need to do something
in clang frontend, or it can only be used in '.ll'.

Thank you.


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